The FGF-5 proto-oncogene specifies a secreted growth factor which bears sequence homology to the prototypic fibroblast growth factors. FGF-5. is expressed in several embryonic settings, in mature brain, and in some neoplastic cells. This application proposes a series of genetic experiments to determine many of FGF-5's distinct functions. We shall disrupt one or both alleles of the FGF-5 gene in murine totipotent embryonic stem (ES) cells by homologous recombination. FGF-5 negative ES cells will be analyzed for differentiation potential as embryoid bodies in vitro and as tumors in vivo. FGF-5 hemizygous ES cells will be injected into blastocysts to derive chimeric mice and establish a disrupted allele in the germ line. Inbreeding will be performed to determine the developmental abnormalities in FGF-5 negative embryos. We shall also establish transgenic mice bearing human FGF-5 genes which are expressed in a subset of the developmentally characteristic sites or which bear structural mutations in a domain of currently unknown function. Mating these transgenic strains with FGF-5 hemizygous mice and backcrossing progeny with the hemizygous strain will generate embryos carrying the transgenes as sole functional allele. The aberrant phenotypes of these embryos will further dissect FGF-5 function. Lastly, we shall generate mice carrying human FGF-5 transgenes mutated at sites known to increase the translation efficiency of FGF-5 MRNA. These mice should overexpress FGF-5 protein without ectopic expression, and resulting abnormalities will also help determine FGF-5 function. These mice shall also be monitored for cancer predisposition.